Fiber optic sensor systems utilize phase-modulated carriers to transmit information of interest in the phase of an optical signal. The carrier is manifested as a sinusoidal phase modulation of the optical wave that is used by a pressure sensitive optical sensor. The pressure information transduced by the optical sensor adds an additional phase modulation to the optical signal. When the optical signal is received at a remote location, usually via fiber optic media, the pressure information is extracted from the optical signal in a process known as demodulation.
Demodulation involves converting the optical signal to an electrical signal. In digitally oriented systems, the analog electrical signal is passed through an analog-to-digital (A/D) converter, after which the desired pressure information may be extracted via digital means. In a Frequency Division Multiplexed (FDM) system, multiple optical carriers are combined through an array of sensors. The resultant electrical signal is very complex. This situation is analogous to an Frequency Modulation (FM) cable system where many carriers or channels are transmitted on a single conductor. When demodulating an FM signal, the conventional process is called heterodyning.
While similar to heterodyning, the conventional process of demodulating an optical sensor system is called homodyning. The process of utilizing homodyning to extract a signal with many carriers is very process intensive. Two mixers are utilized for each of the multiplexed carriers. These mixers down-convert the carrier signal's first and second harmonics to baseband. The mixing process produces numerous undesirable harmonics that must be filtered out. A series of lowpass filtering stages is utilized to recover the desired in-phase and quadrature signal. When such a process is performed on carrier signals in the MHz range, the processor performing the mixing and filtering may be required to perform billions of mathematical operations per second. Although such a process is not impossible, it is difficult to implement such a process with hardware that fits in a small space and consumes low power.
Accordingly, there is a need for an apparatus for and method of demodulating multiple-carrier phase-modulated signals that does not produce unwanted signals while utilizing a reduced amount of computation when compared to known methods.